Method of vacuum-tight closure of thin beryllium windows and x-ray tube provided with such a window

ABSTRACT

The invention provides a method of vacuum-tight closure of beryllium windows of X-ray tubes and especially thin windows for use in X-ray fluorescence analysis of elements of a low atomic number, said windows having a thickness of only about 50 Mu . The window material or the ready windows are for this purpose provided by a known method of sputtering material particles with an extremely thin layer of beryllium or beryllium oxide. The practically unavoidable pores or cracks in the beryllium foil are found to be completely closed by a layer thickness of about 1 Mu . By this method rejects in the production due to bad vacuumtightness can be avoided and it becomes thus possible to manufacture reliable X-ray tubes provided with these extremely thin windows.

United States Patent [721 Inventors Jan Goorlssen;

Pieter Van Der Wei-f, both of Emmasingel,

[54] METHOD OF VACUUM-TIGIIT CLOSURE OF THIN BERYLLIUM WINDOWS AND X-RAY TUBE PROVIDED WITH SUCH A WINDOW 5 Claims, 2 Drawing Figs.

[52] U.S. Cl 313/59, 204/192, 313/55, 313/233 [51] Int. Cl 110lj 35/18 [50] Field of Search 313/55-60,

[56] References Cited UNITED STATES PATENTS 2,663,812 12/1953 Jamison et a1. 313/59 2,866,114 12/1958 l-lardenberg 313/59 Primary Examiner-Roy Lake Assistant Examiner-E. R. LaRoche Attorney-Frank R. Trifari ABSTRACT: The invention provides a method of vacuumtight closure of beryllium windows of X-ray tubes and especially thin windows for, use in X-ray fluorescence analysis of elements of a low atomic number, said windows having a thickness of only about 50 u. The window material or the ready windows are for this purpose provided by a known method of sputtering material particles with an extremely thin layer of beryllium or beryllium oxide. The practically unavoidable pores or cracks in the beryllium foil are found to be completely closed by a layer thickness of about 1 t. By this method rejects in the production due to bad vacuum-tightness can be avoided and it becomes thus possible to manufacture reliable X-ray tubes provided with these extremely thin windows.

12 XMQ PATENTEDN V l9?! 3.617.788

I saw 10F 2 JAN GOORISSEN PIETER VAN DER WERF INVENTORPI AGFNT PATENTEDunv 2 IHTI "3,617,788 SHEET 20F 2 INVENTORS JAN GOORISSEN PIETER VAN DER'WERF METHOD OF VACUUM-TIGHT CLOSURE OF THIN BERYLLIUM WINDOWS AND X-RAY TUBE PROVIDED WITH SUCH A WINDOW For carrying out X-ray fluorescence analysis of materials X- ray tubes are employed which are provided with one or more extremely thin beryllium windows for passing the X-rays produced in the tube. These windows often have a thickness of 100 p. at the most. It is thus ensured that soft X-rays required for fluorescence analysis of a number of materials of interest for the investigation, having an atomic number of less than 12, for example, carbon, oxygen and sodium are fairly satisfactorily passed through the window. It would be preferred to use windows of a thickness of about 50 .1..

The manufacture of strips of metal foil by an appropriate mechanical treatment is difficult especially with beryllium and a considerable portion of the foils are lost due to cracks or pores. Therefore, window plates cut from such metal foil and arranged in the window frame are often not vacuum-tight and hence unserviceable. Reinsertion of a window involves additional time and costs. This is not readily avoidable because a preliminary test of cutout window plates for assessing the vacuum-tightness does not always give reliable results. It is furthermore unavoidable that a piece of metal foil of comparatively small size, having many pores, is divided among a plurality of windows during cutting, which are then unserviceable.

The invention relates to a method of vacuum-tight closure of beryllium windows of a thickness of not more than 100 ,u. for closing exit openings of X-ray tubes and has for its object to obviate said disadvantages. According to the invention the mechanically machined window cut from a thin, rolled plate of beryllium is provided by a particle sputtering technique with a covering layer closing the pores of the window. The effect of this measure on the vacuum-tightness of the treated windows is surprising. A coating of a thickness of about I 41 applied to a beryllium window by sputtering provides a complete closure of the pores provided these pores do not exceed about 1.. By appropriate choice of the material to be applied and by the very small thickness of the layer it is ensured that the X-ray absorption of this layer, even of soft rays is negligible. The material to be sputtered should for this purpose contain only elements of a low atomic number and usually consists of beryllium oxide or beryllium.

Various methods may be used for applying this coating, for example: the rolled beryllium foil may be treated before cutting window plates. The cutout window plates may be treated separately, while the use of material of apparently bad portions of a metal foil can be avoided. The windows already arranged in a frame may be treated, which facilitates handling and checking with respect to vacuum-tightness. Finally the treatment may be carried out after the windows, which may no longer be flat but be provided with some profile for special requirements, have been arranged in the window opening of the X-ray tube.

The invention will be described hereinafter with reference to the drawing, in which FIG. I is a quite schematic sectional view of a device for carrying out the method and FIG. 2 is a sectional view of an X-ray tube provided with a window in accordance with the invention.

In the arrangement of FIG. I the sputtering space 1 communicates through an opening 2 in a base plate 3 with a suction pump (not shown), while the space 1 is bounded by a bell 4, preferably of glass. The sputtering space accommodates a metal electrode 5, to which a beryllium oxide plate 6 is fastened The electrode 5 together with the plate 6 is electrically connected via a through-connection 7 to a high-frequency generator 8, which is capable of supplying a voltage up to 4 kv. of a frequency of 20 MHz. The electrode 5 and the supply conductor 9 inside the bell are screened by an earth-con nected sheath 10. A vapor-deposition table 11, electrically connected to a base plate, can receive the beryllium windows to be treated. The base plate has the further required passages,

the passage 12 for the supply of ar on as to the sputtering space 1 being shown. By a control v ve 2 the pressure of the argon gas can be adjusted in the space 1. This gas pressure is of the order of 5X10 Torr. The high-frequency field applied, together with a magnetic field produced by a magnet coil 14, produces a plasma of argon ions and maintains it so that particles are released from the beryllium oxide plate 6 and applied to the window 15 lying on the table 11. The thickness of the coating is controlled by the duration of the process and is about 1 u. If a coating of beryllium metal instead of beryllium oxide is desired, the plate 6 must be replaced by a plate of beryllium metal, in which case a capacitor must be connected between the electrode 5 and the high frequency source in order to cause the process to be performed in the correct sense. It is furthermore possible to volatilize beryllium in an oxygen atmosphere, so that a coating of beryllium oxide is formed. If the window plate is to be treated after mounting in the X-ray tube, an opening (not shown) has to be provided in the bell or in the base plate, through which the window can be disposed approximately at the place of the then failing table 11. The tube to be treated must then be conductively connected to the base plate.

One embodiment of an X-ray tube comprising a window thus treated is shown in FIG. 2. This tube is of the type disclosed by the applicant in Dutch Pat. application No. 6,708,463, where the desirability of thin windows is indicated and the problem of lack of vacuum-tightness of these windows is involved. The tube comprises a glass sheath 20 enclosing a vacuum space in which a cathode 21 is arranged, opposite which a hollow metal tube 22 is arranged, which extends beyond a seal 23 of the sheath 20 and is made of ferromagnetic material, for example, iron. The end of this tube accommodates a copper anode 24, which closes the tube in an airtight manner, while the window 25 of a thickness of, for example, 50 [-L is located opposite said anode in the wall of the tube through which window the X-rays released at 26 emerge. If it is desired to provide the window 25 only after being mounted in the X-ray tube with a coating, the projecting tube portion may be arranged in the bell of FIG. 1. The small quantity of beryllium or beryllium oxide deposited on the tube wall during treatment will not give rise to any difficulty. If desired, the tube may be screened against it.

What is claimed is l. A method of hermetically sealing beryllium windows of a thickness of less than ,u for closing radiation openings of X-ray tubes comprising the step of sputtering beryllium or beryllium oxide onto a thin rolled sheet of beryllium to form a coating thereon closing the pores in the beryllium sheet.

2. A method as claimed in claim 1 wherein window plates are cut from a foil and are fastened in a frame readily mounted in the exit opening of an X-ray tube, said plates then being provided with a coating on at least one side thereof.

3. A method as claimed in claim 2 wherein the window plate is mounted in an X-ray tube and provided with a coating.

4. A method as claimed in claim 1 wherein the coating has a thickness of about 1 u.

5. An X-ray tube for producing comparatively soft X-rays for fluorescence analysis of elements of a low atomic number comprising an evacuated envelope provided with anode and cathode electrodes, said envelope having a beryllium exit window having a thickness of about 50 11., said window provided on at least one side with a sputtered coating of beryllium or beryllium oxide closing the pores in the window.

* :u a m 

2. A method as claimed in claim 1 wherein window plates are cut from a foil and are fastened in a frame readily mounted in the exit opening of an X-ray tube, said plates then being provided with a coating on at least one side thereof.
 3. A method as claimed in claim 2 wherein the window plate is mounted in an X-ray tube and provided with a coating.
 4. A method as claimed in claim 1 wherein the coating has a thickness of about 1 Mu .
 5. An X-ray tube for producing comparatively soft X-rays for fluorescence analysis of elements of a low atomic number comprising an evacuated envelope provided with anode and cathode electrodes, said envelope having a beryllium exit window having a thickness of about 50 Mu , said window provided on at least one side with a sputtered coating of beryllium or beryllium oxide closing the pores in the window. 